1. Field of the Invention
The invention relates generally to global positioning system (GPS) devices and more particularly to systems and methods for the simultaneous position and/or velocity determinations of a plurality of ground-based units where the ground-based units' positions relative to one another and their absolute positions need to be determined for the same instant, and where the ground-based units have signal acquisition of a common set of GPS satellites. (The term "ground-based" includes anywhere on Earth, e.g., undersea, on-the-sea, on-the-land, and in-the-air.)
2. Description of the Prior Art
The United States of America Department of Defense has placed in orbit a group of satellites, as part of a global positioning system (GPS), that can be used by civilians and the military alike to get automated and highly-accurate earth position coordinates on easy to read digital displays. Determining where you are has been a particular problem for seafarers for thousands of years. Now, GPS enables small sailboat owners and even combat soldiers to get their positions to within several meters using handheld portable equipment.
Ocean and sea-bottom surveying equipment routinely use earth positions to generate geologic studies of earth strata. A typical geologic survey method involves setting off an explosive charge that creates a shockwave and then records the reflections that arrive at several locations in the general area. Using well-known survey techniques, the reflected sound information can be analyzed, and three-dimensional images of the structures can be reconstructed by computer and displayed on a screen. Critical to the analyses are the respective locations of the source of the shockwave and the locations of the individual listening stations. The computer depends on having extremely accurate relative positions for the above. The absolute position of such a group is more important to the users, rather than the computer. The absolute positions are used to pinpoint structures on the map, so that they may be visited again.
Previous systems have made absolute determinations of where the respective groups members are, and then solved from that what the relative orientations are. Each GPS receiver is usually able to acquire the signal of several GPS satellites at once. Normally three range measurements are required to triangulate a location, but four are actually needed with GPS to correct local clocks to GPS system time. The individual distance legs from each GPS satellite to each GPS receiver are therefore called pseudo ranges (PR). Therefore, for four SVs and two GPS receivers, eight PRs will be generated.
The present invention solves the problem of finding the relative distance between GPS receivers by time-tagging candidate PRs, aligning like tagged PRs from respective GPS receivers, and subtracting the difference between corresponding PRs to arrive at highly accurate relative position determinations. This is a so-called "range-domain" approach, rather than the more conventional "position-domain" approach referenced in the previous paragraph.